The present invention relates to ink jet printbars and more particularly to a printbar cooling device. Although suitable for any size printbar, it is particularly useful for a full-page-width array of print dies. Full-page-width arrays of print dies require a constant temperature across the entire width of the array to maintain consistent print quality. Each print die contains an ink manifold providing ink to hundreds of minute capillary channels leading to the print face of the printbar. Each capillary channel is individually activated by heating a portion of the ink within the channel, thereby forming a small steam bubble which displaces and propels ink out of the channel onto a sheet of paper. The size of the steam bubble produced, the volume of ink displaced, and the velocity of the displaced ink are highly dependent upon the temperature at which the ink is maintained within the printbar. Through normal operation of the printbar, however, up to several hundred watts of heat are generated. As a result, thermal gradients across the print die array can cause unacceptable variations in bubble and drop size and, in general, adversely affect the operation of the printbar and the resulting print.
Previous cooling systems have been proposed utilizing a single water channel entering into thermal contact with the print die array at one end and flowing the length of the array to an outlet at the other end. At lower flow rates, however, the injected coolant tends to absorb more heat at the inflowing end and less heat as the coolant increases in temperature as it approaches the outflowing end of the array, causing or failing to correct a significant thermal gradient. Because thermal gradients in excess of .+-.2.degree. C. across the width of the array are undesirable, coolant flow across the length of the array must be increased significantly to minimize the thermal gradient. Increasing the flow rate, however, requires larger, more expensive coolant pumps. In addition, higher flow rates render the system more prone to leakage.